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Chapter 2: Movement of Substances (DIFFUSION (Rate of diffusion is…

the net movement of water molecules from a solution of higher water potential to a solution of lower water potential through a partially permeable membrane

water potential

a measure of the tendency of water to move from one place to another. A dilute solution contains more water molecules per unit volume than a concentrated solution, so it has a higher water potential than a concentrated solution.

DIFFUSION

the net movement of particles from a region of higher concentration to a region of lower concentration down a concentration gradient. Concentration gradient is the difference in concentration between two regions

the steeper the concentration gradient, the faster the rate of diffusion

Diffusion across a membrane

the concentration of the particles from both side will eventually be equal. This is called equilibrium

The cell surface membrane allows only some substances to pass through such as oxygen and carbon dioxide. An example is the lungs. The cells in the lungs exchange oxygen and carbon dioxide through diffusion.

Rate of diffusion is affected by:

Temperature.

Surface area

Concentration gradient

Size of particles

Diffusion medium

OSMOSIS affecting living organisms

plant cells

if placed in a solution with higher water potential(hypotonic solution), the cell will expand and become turgid

if placed in a solution with lower water potential(hypertonic solution), the cell will shrink and become flaccid or plasmolysed

animal cells

if placed in a solution with higher water potential(hypotonic solution), the cell will expand and burst.

if placed in a solution with lower water potential(hypertonic solution), the cell will shrink and become crenated

SURFACE AREA TO VOLUME RATIO

the rate of movement f a substance across the surface of a cell depends on how big the cell surface membrane area is. The greater the area of cell surface per unit volume, the faster the rate of diffusion of a substance for a given concentration gradient.

increase in surface area, decreases the volume

adaptation of cells to their surface area for absorption

For example, the root hair cell has long narrow protrusion or folds in its membrane. These greatly increases the surface area of cell surface membrane through which materials can be absorbed into the cell.

ACTIVE TRANSPORT

the process in which energy is used to move the particles of a substance across a membrane against its concentration gradient, that is, from a region of lower concentration to a region of higher concentration